We will study two intestinal epithelial cell types that are specialized for the uptake of luminal macromolecules: the absorptive cell of neonatal ileum, and the"M" cell in Peyer's patch. Our general aim is to define the strategies used by these cell in vivo to deal with foreign proteins and particles that bind to their luminal plasma membranes. We will visualize these strategies by following the intracellular fates of ultrastructurally-tagged ligands and ligand-coated beads in their internal membrane systems, using neonatal and adult rats. The neonatal ileal cell is specialized for intracellular digestion of macromolecules, and contains and "endocytic complex" consisting of multiple membrane compartments. We will determine whether soluble and membrane bound proteins are "sorted" here for differential transport and whether certain ligands are transported intact across the epithelium. The distribution and mobility of binding sites for specific ligands (lectins, cationized ferritin) on the luminal membranes of living ileal cells will be defined, and the precise routes of these ligands through the endocytic complex will be followed. Lysosomal enzyme activity and pH in various endocytic compartments will be related to ligand transport, and the system will be manipulated by raising intracellular pH or by removing the extralysosomal saccharidase present on the ileal cell's endocytic membranes. The M cell is known to conduct transepithelial transport, but whether it uses multiple transport systems for soluble proteins, membrane ligands and particles isn't known. We will define these transport systems by applying combination of well-defined tracers and microspheres to Peyer's patch epithelium. Particle ingestion will be correlated with rearrangements of cytoskeletal elements, as visualized by ultrastructrual S1-decoration and LM immunocytochemistry, to distinguish between phagocytosis and endocytosis in these cells. Our results will demonstrate directly how individual intestinal cells handle specific types of membrane-bound lectins and other ligands. This data will allow us to predict which types of dietary or bacterial surface proteins are most likely to "trigger" transepithelial transport and succdeed in crossing the intestinal epithelial barrier in antigenic or pathogenic amounts.